Leakage and performance of zero-Schottky-barrier carbon nanotube transistors

Abstract

Undoped carbon nanotube field-effect transistors (CNTFETs) with zero-Schottky-barrier contacts have the current-voltage response of field-effect transistors (FETs), but the physics of their operation is that of voltage-controlled tunnel barriers. The carbon nanotube (CNT) body itself provides the tunnel barrier. The leakage current in CNTFETs is a combination of both interband and intraband tunneling and this current can be significantly reduced by changing the CNT diameter as well as the CNT length and source/drain asymmetry. Source and drain extensions significantly reduce the leakage current and increase the ON/OFF current ratio. Asymmetry with the gate closer to the source further reduces leakage, improves the ON/OFF current ratio, decreases the switching time, and increases the cutoff frequency despite the higher gate capacitance. An ON/OFF current ratio of >104 can be obtained from a 50-nm-long, 1.5-nm-diameter CNT with a 2nm gate. The switching time is very small in the 0.1ps range and the cutoff frequency is very high in the 4THz range. Coulomb blockade is expected to block the interband resonant tunneling (ambipolar) leakage current so that the CNTFETs become effectively unipolar devices. Poisson’s equation is solved self-consistently with the nonequilibrium Green’s-function equations using a π-bond model for the CNT.